In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Serving Gateways (SGWs), or Mobility Management Entity (MME) rather than to radio network controller (RNC) nodes. In LTE, the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE), MME and SGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Cellular Circuit-Switched (CS) telephony was introduced in the first generation of mobile networks. Since then CS telephony has become the largest service in the world with approximately 4 billion subscriptions sold. Even today, the main part of the mobile operator's revenue comes from the CS telephony service (including Short Message Services (SMS)), and the 2G GSM networks still dominate the world in terms of subscriptions, with 3G subscriptions increasing in volume.
The long-term evolution (LTE) project within 3GPP aims to further improve the 3G standard to, among other things, provide even better mobile broadband to the end users (higher throughput, lower round-trip-times, etc.).
By employing shared pipe and packet data scheduling, LTE offers many advantages over the previous 2G and 3G technologies. These include better usage of the available spectrum, much higher data rate, lower latency and simplified network architecture. Although LTE is a wireless technology optimized for packet data transfer, it can also be used to deploy traditional CS-domain services such as voice and SMS. Delivering voice and SMS services over a LTE network requires not only LTE access network, IMS and IP core networks are also essential. During the early stage of LTE roll-out, voice service is not supported by LTE natively. The standards bodies have provided solutions for traditional CS-domain services to coexist with LTE.
However, due to the nature of packet data, LTE has to fall back to 2G or 3G to handle circuit switched voice calls. One solution for supporting voice is called Circuit-Switched (CS) Fallback. This allows an LTE device to drop back to the legacy 2G/3G network when IMS VoLTE capabilities are not in place. The UE normally camps on the LTE network, and must “fallback” to a 2G or 3G network to use the CS domain in order to receive or place a voice call.
Unfortunately, during Circuit Switch Fall Back (CSFB), the LTE connection has to be released. This may cause some undesired outcomes. For instance, when CSFB occurs during an on-going FTP session, the FTP session will be torn down. When the LTE connection resumes, the FTP session needs to start over. A large amount of previously downloaded/uploaded could be lost or wasted.
There are 2 issues associated with releasing the LTE connection. The first is associated with the tearing down of the LTE connection, which can be problematic for applications such as FTP and gaming due to total loss of a FTP or gaming session. The second is the extra time required to bring down the LTE connection, which may introduce excessive delay in setting up the CS domain service, which leads the end user to believe the set-up has failed, thus gives up the attempt pre-maturely. Human behavior often causes a call to be de-activated after a prolonged delay and especially when the end user gives up the call attempt before the called party has time to respond.
Some scenarios are envisaged wherein a wireless terminal camps on both the LTE network and the 2G or 3G network. Such a scenario is described in applicant's co-pending patent applications WO2011/073884, WO2011/073849 and WO2011/073910. However, the ability to maintain parallel registration in two networks is not feasible in some networks and by some wireless terminals. In fact, some devices and networks cannot maintain both a packet-switched connection and a circuit-switched connection simultaneously.
Accordingly, a need exists for a method and system to retain a packet switched session on an LTE core network for a pre-defined period of time while a wireless terminal switches over to a CSFB session.